Here “sap and leaf and wood” refers to every cell of a tree. The poet wants to say that the tree planter’s loyalty and love for this big worldly home, his far-reached thought of civic good and his blessings on his neighbourhood are felt or reflected in the sap and leaf and wood of the tree — in every cell of the tree which he plants.

He plants in sap and leaf and wood,
In love of home and loyalty
And far-cast thought of civic good—
His blessings on the neighbourhood,